1. Field of the Invention
The present invention relates to a contact type image sensor which can be effectively used as a photoelectric conversion device in facsimile machines and others. More particularly, the present invention relates to a low noise contact type image sensor using a plurality of chargetransfer device (CTD) shift registers.
The invention also relates to a method of driving such a image sensor.
2. Description of the Related Art
Conventionally, facsimile machines have been constructed with a small linear image sensor of the MOSIC type or the charge coupled device (CCD) type which has an active length extremely smaller than the width of a document to be read. In such facsimile machines, in order to form an image of the document in a reduced manner on the image sensor, it has been required to locate between the image sensor and the document a reduction optical system having an optical path of a substantial length, so that the machine will inevitably be of a large size. Recently, full document width contact type linear image sensors have been developed. Since the contact type linear image sensor needs only a very short optical path between the image sensor and the document, and does not require any reduction optical system, the facsimile machine can be fabricated with a more compact size and at a lower cost than the conventional machine.
However, the contact type image sensor itself is of a large size because it includes a photodetector array having the same active length as the width of the document to be read. For example, a contact type image sensor for a A-4 size document has to have an active length of 216 mm. In addition, if it needs the resolution of 8 or 16 picture elements/mm, the image sensor is required to have 1728 or 3456 photodetector cells.
Referring to FIG. 1, there is shown one conventional linear image sensor. This linear image sensor is in the form of a hybrid circuit fabricated on a glass substrate. Specifically, the image sensor comprises a large number of photodetectors 1 made of, for example, amorphous silicon and formed on the glass substrate in a straight line. It further includes a plurality of driving integrated circuit chips 2a, 2b, . . . and 2n all located on the same glass substrate. Each of the driving IC chips 2a, 2b, . . . 2n includes a scanning pulse generating circuit 3a, 3b, . . . 3n constituted of, for example, a shift register of 64 or 128 stages, and a number of gate switches 4 constituted of one MOSFET adapted to be turned on and off by the corresponding one stage of the scanning circuit. These gate switches 4 are also connected at its one end to the respective photodetectors 1 in a one-to-one relation and at its other end to a common signal line 5. A clock line 6 is connected to an a clock input of each scanning circuit, and each scanning circuit is connected at its output to an input of the next scanning circuit. Thus, in the condition that a clock .phi. is supplied through the line 6 to the clock input of each scanning circuit, if a start pulse .phi..sub.s is applied to an input 7 of the first scanning circuit 3a; the MOSFET gates 4 are sequentially turned on, so that a photosignal stored in the respective photodetectors 1 is read out to the signal line 5. Namely, the image sensor operates in a storage mode.
In such a reading-out, the following two noises are generated: The first is a switching noise generated in the form of a spike by the leakage of the gate driving pulse through a gate-drain capacitance 8 of the MOSFET switch 4. The second noise is a clock noise generated through a parasitic capacitance 9 between the clock line 6 and the signal line 5.
As a light source for the contact type image sensor, a light emitting diode (LED) is widely used. The LED is, however, limitative in the intensity of the emitted light. Therefore, in the case of reading the photosignal at a scanning speed of 5 to 10 msec/line, which is generally required in the facsimile equipment, or at a higher speed, the photosignal obtained is too weak and therefore the signal-to-noise (S/N) ratio is not so high. As the result, noise suppression has been required in order to obtain a sufficient S/N ratio.
For this purpose, Koike et al proposed one noise suppression method in "Improvements of S/N Ratio of MOS Image Sensor by Neighboring Bit Correlation Method", Transactions of Institute of Electronics and Communication Engineerings of Japan, Vol. J 60-C, pages 113 to 120 (1977). As shown in FIG. 2, a pair of MOSFETs 21a and 21b is connected to each photodetector 22, and one of the MOSFETs 21a in each pair is connected to a noise line 23 while the other MOSFET 21b is connected to a signal line 24. In addition, each scanning pulse output stage of the scanning circuit 25 is connected to the gate of the MOSFET 21b which is coupled to one photodetector 22.sub.i and also to the gate of the MOSFET 21a which is coupled to one photodetector 22.sub.i-1 just before the photodetector 22.sub.i in the scanning direction.
With this arrangement, each two adjacent MOSFET switches 21a and 21b respectively connected to each pair of adjacent different photodetectors 22.sub.i-1 and 22.sub.i are simultaneously turned on by a scanning pulse supplied from the scanning circuit 25, so that a signal including a noise is read out on the signal line 24 and only a noises appears on the noise line 23. Therefore, the noise component can be removed from the voltage on the signal line 24 by obtaining the difference in voltage between the lines 23 and 24. Thus, there can be suppressed noises having a fixed pattern, such as the aforementioned switching noise and clock noise.
Turning to FIG. 3, there is shown a second method for noise suppression proposed by Takamura et al. in "Low Noise Linear MOS Image Sensor", National Technical Report, Vol. 21. No. 6, pages 692 to 703 (1975). In this sensor, each photodetector 31 is connected through one MOSFET switch 32 to a signal line 33, and instead of the dummy MOSFET 21a shown in FIG. 2, a MOS capacitor 34 having the same capacitance as the MOSFET switch 32 is connected to each MOSFET switch and a noise line 35. One MOSFET switch 32 and the MOS capacitor 34 connected thereto are simultaneously switched on and off by a scanning circuit 36, so that a signal including a noise is outputted to the signal line 33 and a noise only on the noise line 34, similarly to the lines 23 and 24. The noises can therefore be removed from the signals by obtaining the difference of voltage between the lines 33 and 35.
Besides, Ohba et al. proposed a third method for noise suppression in "Fixed Pattern Noise In an Area Sensor And FPN suppressing Circuit", Television Society Technical Report, Vol. 4. No. 13, pages 53 to 58 (1980). In this method, signal and noise are integrated during the driving period for each photodetector cell so that only a pair of positive and negative noises caused by one scanning pulse are cancelled each other.
Saito et al., moreover, proposed a fourth method in "A-4 16 bits/mm Contact Type High Speed Image Sensor Using an a-Si:H Photodetector Array", Technical Report of Institute of Electronics and Communication Engineers of Japan. ED 83-64, October, 1983. In this method, the gate switch 4 is constituted by a CMOS switch consisting of a pair of series-connected p-channel MOSFET and N-channel MOSFET. A pair of pulses reverse in phase to each other are applied to the respective MOSFETs of the CMOS gate switch, so that noises of opposite polarities to each other appear in the pair of MOSFETs, respectively, but are resultingly cancelled by each other.
Thus, there has recently developed a contact type image sensor for A-4 size which has 16 elements/mm resolution and a S/N ratio of 20 dB at 0.8 msec/line scanning speed. However, any of the above-mentioned various noise suppression methods cannot remove the noise over a certain degree, because of nonuniformity in characteristics of each MOSFET and the clock noise. Because of this, if the image sensor is driven at a speed higher than 0.5 msec/line, the S/N ratio and the resolution become lower. In addition, it is recently desired to reproduce a half tone image in the facsimile. For this purpose, a S/N ratio higher than 40 dB is required for example. Therefore, in the aforementioned noise suppression methods are sufficient to give the required performance to the image sensor.
In the above mentioned methods for driving the linear image sensor, the photodetectors are sequentially read out by the scanning circuit, and therefore, the noise is generated at each reading-out so that the noise cannot be sufficiently suppressed. In order to overcome the problem of the sequential reading of the photodetectors, there has been proposed and used a CCD linear image sensor as shown in FIG. 4. Photodetectors 41 which store electric charges of photosignals are connected through transfer gates 42 to respective stages of a CCD shift register 43. These transfer gates 42 are simultaneously turned on by a transfer pulse .phi..sub.G as shown in FIG. 5, so that a photosignal in each photodetector 41 are simultaneously transferred through the associated gate 42 to the corresponding stage of a CCD shift register 43. Then, the photosignal in each stage of the CCD shift register 43 is serially transferred stage by stage by alternately supplying clock pulses .phi. and .phi. as shown in FIG. 5, so that a photosignal SP as shown in FIG. 5 is outputted from the CCD shift register 43 to an output amplifier 44.
In the reading operation of the CCD image sensor as mentioned above, so-called "feed-through" noise N.sub.f appear in the photosignal as shown in FIG. 5. But, this noise is generated when the transfer gates 42 are triggered. Therefore, the feed-through noise is a fixed pattern noise, and will not appear during the period of time for reading the photosignal of each scanning line from the CCD shift register. Accordingly, it is easy to remove the feed-through noises in a subsequent process of the signals. As seen from the above, the CCD image sensor is free from the fixed pattern noise, as generated in the MOSIC image sensor as shown in FIGS. 1 to 3.
In CCD image sensors recently developed, moreover, it is able to drive the CCD shift register at a transfer clock pulse frequency of 10 MHZ or higher. If such a high speed CCD image sensor is applied to a contact type image sensor for A-4, size having 16 elements/mm resolution, namely, having 3456 photodetector cells, a reading speed of 0.4 msec/line or more can be expected. In addition, by alternately reading photodetector cells arranged in a line by use of a pair of CCD shift registers at a speed of 0.4 msec/line, a speed of 0.2 msec/line or more can be realized.
Furthermore, if a so-called floating gate amplifier is used as the output amplifier 44, the CCD image sensor can have an increased sensitivity, so that it can obtain an S/N ratio higher than the conventional MOSIC image sensor.
However, when there is fabricated a CCD linear image sensor having for example 1728 or 3456 photodetector cells, since a CCD shift register chip having 1728 or 3456 stages is not commercially available, it is necessary to use a plurality of CCD shift register chips. In such a case, when a photosignal for one scanning line is read out by sequentially scanning the respective CCD shift registers, the feed-through noise N.sub.f will inevitably appear at the boundary between the photosignal from one CCD shift register and the photosignal from the next CCD shift register. Therefore, a photosignal for one scanning line will inevitably contain several feed-through noise N.sub.f.
In addition, in the case that each of the CCD shift registers is provided at its output with one floating gate amplifier, if all the floating gate amplifiers are directly connected together to a common output line of the image sensor, a short-circuit situation is caused.
Therefore, there is necessity for providing output terminals of the same number as the CCD shift registers used. For example, if each CCD shift register is of 256 stages, a contact type image sensor for A-4 size having 8 or 16 elements/mm resolution needs 7 or 14 output terminals. Such a CCD image sensor becomes complicated in wiring as compared with the MOSIC image sensor, resulting in poor reliability of connection to the external.
Furthermore, the CCD image sensor has a problem of a so-called "image lag". Namely, when the photosignal is small, specifically when the voltage across the photodetector 41 is not greater than 0.3 V, even if the transfer gate 42 is turned on, the electric charge will not be quickly and sufficiently be transferred from the photodetector to the CCD shift register. The voltage across the photodetector is determined by the amount of a photoelectric charge stored in the photodetector and the capacitance of the photodetector. Namely, the voltage across the photodetector is increased by increase of the amount of the photoelectric charge stored in the photodetector, but decreased by the capacitance of the photodetector. In addition, if the capacitance of the photodetector becomes larger than that of each stage of the CCD shift register, the amount of the electric charge transferred from the photodetector to the CCD shift register becomes small.
However, it is actually difficult to make 1 pF or less of capacitance of each photodetector including a capacitance of the associated distributing wire et al. On the other hand, in the case that a contact type image sensor for A-4 size having 16 elements/mm resolution and using a yellow green (wavelength 570 nm) LED having 100 lx in illuminance at the sensor surface is driven at the scanning speed of 1 msec/line, the photoelectric charge is merely about 0.2 pC per photodetector. As a result, the voltage across the photodetector will be only 200 mV. Additionally, the capacitance of the photodetector such as a-Si type photodetector cell is generally larger than that of the CCD type image sensor. Therefore, in the case that a-Si photodetectors are directly connected to the CCD shift register, the above mentioned image lag will occur, and therefore, a sufficient sensitivity cannot be obtained.